Performance of Thermoelectric Generation Module Using Mg₂(Si,Sn) Based Thermoelectric Materials

Thermoelectric conversion is an energy conversion technology that has been around for a long time, which includes thermoelectric power generation that can directly convert heat energy into electrical energy in principle without emitting CO₂, and thermoelectric cooling and temperature control that can directly convert electrical energy into heat energy. Thermoelectric conversion technology is expected to help address environmental issues believed to be caused by global warming, as well as the increasing energy demand issue. In the concept of a rapidly expanding smart society in recent years, there are many challenges that need to be addressed, and stable energy supply is one of them. In addition to the use of natural energy sources such as wind power and solar power, effective utilization of renewable energy sources like waste heat utilization for thermoelectric power generation is also considered as one of the solutions to the energy supply problem. Furthermore, there is a growing demand for energy harvesting as a power source for driving various sensors that are widespread in modern society. Thermoelectric power generation is a versatile technology that can be applied regardless of the scale, from powering mW-level wireless sensor drives to MW-level large power generation.<br/>While there have been many research efforts to develop and improve the performance of thermoelectric materials, the practical thermoelectric conversion materials currently in use are mainly Bi₂Te₃-based and PbTe-based, which contain heavy metals, and are still relatively few compared to the long history and extensive research on materials. When selecting thermoelectric materials, it is important to consider not only having high thermoelectric performance but also factors related to safety (non-toxicity), thermal stability, and peripheral technologies related to module development such as device technology for modularization. In addition, for the practical implementation of thermoelectric conversion technology, further enhancement of material performance, scaling up of modules, establishing mass production processes, confirming the durability (reliability) of modules and systems, reducing costs, and providing numerous examples of successful applications are necessary.<br/>KELK Ltd. and Komatsu Ltd. have been focusing on Mg₂(Si,Sn) compounds that demonstrate relatively high thermoelectric characteristics in the temperature range from around 300°C to 600°C, and have been consistently conducting research from material performance improvement to module development over the years. The figure of merit <i>ZT</i> of the materials obtained so far were 1.2 for N-type materials and 0.5 for P-type materials at around 400°C, where <i>Z</i> is given as <i>S</i>²/(ρκ), <i>S</i> is the Seebeck coefficient (Thermopower) [V/K], ρ is the electrical resistivity [Ωm], and κ is the thermal conductivity [Wm^-1K^-1]. Note that a <i>ZT</i> value exceeding 1 is considered as required potential for practical applications. Furthermore, from the power generation test of modules using these materials, a maximum output of 7.8 W and an output density of 1.2 W/cm² were obtained at the temperature difference Δ<i>T</i> of 470°C (Δ<i>T</i> = <i>T</i>_h500°C/<i>T</i>_c30°C, where <i>T</i>_h and <i>T</i>_care hot-side and cold-side temperature), with a maximum thermoelectric conversion efficiency of 7.5%. The presentation will introduce the results in material research, the current status and challenges of module development.